The present invention relates to methods, components and apparatus useful in mounting and connecting semiconductor devices.
Semiconductor chips typically are connected to external circuitry through contacts on a surface of the chip. The contacts on the chip typically are disposed in the regular patterns such as a grid substantially covering the front surface of the chip, commonly referred to as an xe2x80x9carea arrayxe2x80x9d or in elongated rows extending parallel to and adjacent each edge of the chip front surface. Each contact on the chip must be connected to external circuitry, such as the circuitry of a supporting substrate or circuit panel. Various processes for making these interconnections use prefabricated arrays of leads or discrete wires. For example, in a wirebonding process, the chip is physically mounted on the substrate. A fine wire is fed through a bonding tool. The tool is brought into engagement with the contact on the chip so as to bond the wire to the contact. The tool is then moved to a connection point of the circuit on the substrate, so that a small piece of wire is dispensed and formed into a lead, and connected to the substrate. This process is repeated for every contact on the chip.
In the so-called tape automated bonding or xe2x80x9cTABxe2x80x9d process, a dielectric supporting tape, such as a thin foil of polyimide is provided with a hole slightly larger than the chip. An array of metallic leads is provided on one surface of the dielectric film. These leads extend inwardly from around the hole towards the edges of the hole. Each lead has an innermost end projecting inwardly, beyond the edge of the hole. The innermost ends of the leads are arranged side by side at spacing corresponding to the spacings of the contacts on the chip. The dielectric film is juxtaposed with the chip so that hole is aligned with the chip and so that the innermost ends of the leads will extend over the front or contact bearing surface on the chip. The innermost ends of the leads are then bonded to the contacts of the chip, as by ultrasonic or thermocompression bonding. The outer ends of the leads are connected to external circuitry.
In a so-called xe2x80x9cbeam leadxe2x80x9d process, the chip is provided with individual leads extending from contacts on the front surface of the chip outwardly beyond the edges of the chip. The chip is positioned on a substrate with the outermost ends of the individual leads protruding over contacts on the substrate. The leads are then engaged with the contacts and bonded thereto so as to connect the contacts on the chip with contacts on the substrate.
The rapid evolution of a semiconductor art in recent years has created a continued demand for progressively greater numbers of contacts and leads in a given amount of space. An individual chip may require hundreds or even thousands of contacts, all within the area of the chip front surface. For example, a complex semiconductor chip in current practice may have a row of contacts spaced apart from one another at center-to-center distances of 0.5 mm or less and, in some cases, 0.15 mm or less. These distances are expected to decrease progressively with continued progress in the art of semiconductor fabrication.
With such closely-spaced contacts, the leads connected to the chip contacts, such as the wires used in wirebonding leads in the tab process and beam leads must be extremely fine structures, typically less than 0.1 mm wide. Such fine structures are susceptible to damage and deformation. With closely spaced contacts, even minor deviation of a lead from its normal position will result in misalignment of the leads and contacts. Thus, a given lead may be out of alignment with the proper contact on the chip or substrate, or else it may be erroneously aligned with an adjacent contact. Either condition will yield a defective chip assembly. Errors of this nature materially reduce the yield of good devices and introduce defects into the product stream. These problems are particularly acute with those chips having relatively fine contact spacings and small distances between adjacent contacts.
It has been proposed to form a prefabricated lead assembly having inwardly projecting leads with all of the inner ends of the leads connected to a common inner element. The common element typically is a metallic ring-like structure. In these structures, the inner end of each lead is connected to the common element via a frangible section. The common element thus restrains the inner ends of the leads against relative movement and hence inhibits bending or other deformation of the leads. After the leads have been bonded to the chip contact, the common element is broken away from the leads. A frangible section may be provided at the juncture between the innermost end of each lead and the inner element. Systems of this nature are illustrated, for example, in Thorpe, Jr. U.S. Pat. No. 4,756,080 and in Angelucci, Sr. et al, U.S. Pat. No. 4,380,042. Burns, U.S. Pat. Nos. 4,312,926 and 4,413,404 depict a generally similar arrangement in which the leads are multilayer metallic structures including a copper base with an overcoat of nickel. The frangible connection between the innermost end of each lead and the inner element consists solely of the nickel overcoat layer, thereby providing a very thin, weak section.
In these arrangements, the common element electrically interconnects all of the leads. These interconnections must be eliminated after the leads have been bonded to the chip. Thus, the common element must be pulled away from the chip after the leads have been bonded to the contacts of the chip. All of the frangible elements must be broken either simultaneously or in a particular pattern as the common element is pulled away from the innermost ends of the leads. The need to remove the common element constitutes a significant drawback, inasmuch as this must be done without disturbing the delicate bonds between the lead ends and the contacts on the chip. Perhaps for these reasons, systems utilizing a common element have not been widely adopted.
Thus, despite the substantial time and effort devoted heretofore to the problems associated with mounting and connecting of semiconductors, there have still been substantial, unmet needs for improvements in such processes and in the equipment and components used to practice the same.
The present invention addresses these needs.
One aspect of the present invention provides a semiconductor chip mounting component. A component according to this aspect of the invention includes a support structure having upper and lower surfaces and having a gap extending through the support structure, so that the gap extends downwardly from the upper surface to the lower surface. The component also includes at least one electrically conductive lead. Each such lead has a connection section extending across the gap in the support structure. First and second ends of the connection section are secured to the support structure on opposite sides of the gap. The second end of each connection section is secured to the support structure so that the second end can be displaced downwardly relative to the support structure responsive to a downward force applied to the connection section. Each connection section is flexible, so that the connection section can be bent downwardly when the second end of the connection section is displaced downwardly relative to the support structure. Thus, the connection section of each lead will be supported at both ends by the support structure during positioning of the component on a semiconductor chip assembly. However, each connection section can be bent downwardly to engage a contact on a part of the semi-conductor chip assembly after the component has been positioned on the part.
Most preferably, the connection sections of the leads are connected to the support structure so that the first end of each such connection section is permanently connected to the support structure, whereas the second end of each such connection section is detachable from the support structure upon application of a downward force to the connection section. The first end of each connection section typically is connected, by a further portion of the lead, to a terminal mounted on the support structure.
In a typical arrangement, the component is adapted to be positioned on the chip itself. Thus, when the component is positioned on the chip, the connection sections of the leads will overlie contacts on the chip. The connection sections are bonded to the contacts on the chip. The leads may have terminals remote from the connection sections for connecting the leads, and hence the contacts of the chip, to contacts on a substrate. In the reverse arrangement, the component according to this aspect of the invention may be adapted for positioning on the substrate, with the connection sections of the leads overlying the substrate so that the connection sections can be bonded to the contacts of the substrate. The leads may be connected to the contacts on the chip through terminals remote from the connection sections.
Each lead may include a second end securement section attached to the support structure and a frangible section connecting the second end of the connection section with the second end securement section, so that the second ends of the connection sections are attached to the support structure through the frangible sections of the leads. The frangible sections can be broken upon downward displacement of the connection sections. The frangible section of each such lead may have a cross-sectional area smaller than the cross-sectional area of the second end securement section and smaller than the cross-sectional area of the connection section.
In one preferred arrangement, each lead includes a relatively thick structural metal layer and a relatively thin first supplemental metal layer. The connection section and the second end securement section of each lead incorporate the structural metal layer, whereas the frangible section of each lead includes the first supplemental metal layer but omits the structural metal layer. In another arrangement, the second end of each connection section may be bonded to the support structures so that the bond may be broken upon downward displacement of the connection section. The first end of each such connection section may also be bonded to the support structure. Desirably, the bond between the first end of each connection section is more resistant to breakage upon downward displacement of the connection section than the bond between the second end and the support structure. For example, the bonded area of the first end securement section may be larger than the bonded area of the second end securement section.
In another arrangement, the frangible section of each lead may include a polymeric material. In yet another arrangement, each lead may extend only partially across the gap in the support structure. The second end of the connection section of each lead may be disposed remote from the support structure. The component may incorporate a polymeric strip associated with the connection section of each such lead and extending co-directionally with the connection section entirely across the gap. Each such polymeric strip may be secured to the support structure on both sides of the gap and the connection section of each lead may be bonded to the associated polymeric strip. In this case, the second end of each lead is secured to the support structure only through the associated polymeric strip, and the lead can be displaced downwardly relative to the support structure with breakage or elongation of the polymeric strip.
The support structure may incorporate a polymeric cover layer in proximity to the leads, and the polymeric strips may be formed integrally with the polymeric layer of the support structure. This arrangement provides a polymeric strip overlying the connection section of each lead. As further discussed hereinbelow, the polymeric strip serves to reinforce the connection section of the lead. According to yet a further aspect of the invention, similar reinforcement can be applied even in components which do not incorporate the specific arrangement of connection sections supported at both ends by the support structure as discussed above. According to this aspect of the invention, a mounting component may include a flexible, continuous polymeric reinforcement in contact with each lead at an edge of the support structure so that the polymeric reinforcement will inhabit stress concentration in the lead at such edge when the lead is bent downwardly to engage a contact. Most preferably, the polymeric reinforcement associated with each lead includes a polymeric strip overlying the connection section of the lead. Desirably, the polymeric strips associated with the various leads are integral with a polymeric layer of the support structure.
Most preferably, the support structure is formed from dielectric materials such as polymeric materials, so that the support structure does not electrically interconnect the leads with one another. The support structure may have appreciable thickness, i.e., an appreciable distance between its upper and lower surfaces. The leads may be disposed at an appreciable distance above the lower surface of the support structure. Thus, each connection section is supported above the front surface of the chip or substrate by the support structure before such connection section is displaced downwardly to engage a contact. For example, the support structure may include a plurality of layers with a top layer defining the upper surface of the structure and a bottom layer defining the lower surface. The leads may be disposed above the bottom layer. The component may include terminals disposed on the support structure. In a particularly preferred arrangement, the terminals, as well as the leads are disposed above the bottom layer and the bottom layer is resilient so as to permit downward displacement of the terminals.
The gap in the support structure may be formed as an elongated slot. The connection sections of many leads may extend across such slot. The connection sections extending across each such slot are disposed in side-by-side, substantially parallel arrangement. Some of the leads associated with each such slot may have their first ends disposed at a first edge of the slot and a second edge of the slot, whereas the remaining leads associated with the same slot may have the reverse arrangement, i.e., the first end of the lead disposed at the second edge of the slot and the second end of the lead connection section disposed at the first edge of the slot. This arrangement is particularly useful where some of the leads associated with each slot are connected to the terminals disposed on one side of the slot, whereas others are connected to terminals disposed on the opposite side of the slot. Such an arrangement can be used, for example, with semiconductor chips having elongated rows of contacts.
Alternatively, the gaps in the support structure may be relatively small holes extending through the support structure. One lead, or a few leads, may extend across each such hole. There may be numerous holes disposed at various locations on the support structure. For example, the holes, and the leads, may be disposed in an array substantially covering the top and bottom surfaces of the support structure as, for example, where the component is to be used with a chip or other element having contacts in a xe2x80x9carea arrayxe2x80x9d on substantially the entirety of its front surface.
A further aspect of the invention provides methods of making connections to contacts on a part of a semiconductor chip assembly, such as to contacts on a front surface of a semiconductor chip or contacts on a chip mounting substrate. Methods according to this aspect of the invention desirably include the steps of juxtaposing a connection component, such as a component described above, with the part so that a bottom surface of the support structure in the component faces downwardly, towards the surface of the part and the top surface of the connection component faces upwardly, away from the front surface of the part. The connection component is juxtaposed with the part so that each contact on the part surface is aligned with a gap in the support structure and so that connection sections of leads extending across the gap are disposed above the contacts. The support structure supports each connection section at both sides of each such gap during the juxtaposing step, so that the connection section does not tend to bend or deform at this stage of the process.
The method desirably further includes the step of bonding each connection section to a contact on the part by displacing each connection section downwardly so as to displace one end of each such connection section downwardly relative to the support structure and bring the connection section into engagement with the contact of the part. Preferably, the bonding step is performed so as to detach one end of each connection section from the support structure during the downward displacement of the connection section as, for example, by breaking a frangible portion of each lead or detaching a bond between the lead and the support structure as discussed above. The bonding step most preferably includes the step of engaging each connection section with a recess in a bonding tool so that the bonding tool at least partially controls the position of the connection section in lateral directions transverse to the downward travel of the bonding tool.
The use of the bonding tool to guide and constrain the lead during the bonding step may be applied even where the connection component does not have the connection sections connected to the support structure at both ends. Thus, the step of guiding the connection section of the lead with the bonding tool may be employed even where the leads are cantilevered from an edge of the support structure. Most preferably, the methods according to this aspect of the invention further include the step of aligning the bonding tool with the contacts on the part, such as with contacts of a semiconductor chip. Preferably, the step of engaging the bonding tool with the leads is performed so that the bonding tool actually brings the leads into alignment with the contacts. That is, the contact sections of the leads may be slightly out of alignment with the contacts, but the bonding tool moves the leads in directions transverse to the leads as the bonding tool is engaged with the leads, thereby bringing each lead into alignment with the contacts. Thus, it is unnecessary to achieve exact alignment between the connection sections of the leads and the contacts on the part when the connection component is first applied to the part. Any slight misalignment will be corrected by action of the bonding tool.
Typically, each connection section is an elongated, strip-like structure and the bonding tool has an elongated groove in its bottom surface. The bonding tool is positioned above each contact so that the groove extends in a pre-selected groove direction and extends across the top of a contact. The connection sections of the leads extend generally parallel to the groove direction, so that when the bonding tool is advanced downwardly to engage the lead, the connection section of each lead is seated in the groove. If the lead is slightly out of alignment with the groove, the lead will be moved in lateral directions, transverse to the groove, until it seats in the groove and thus becomes aligned with the contact.
Yet another aspect of the present invention provides a tool for bonding leads to contacts on a semiconductor chip, substrate or other part of a semiconductor chip assembly. A tool according to this aspect of the invention desirably includes a generally blade-like body defining a bottom edge and a groove extending lengthwise along such bottom edge for engaging leads to be bonded. The tool desirably also includes means for connecting the tool to a bonding apparatus so that the bottom edge of the tool faces downwardly. Such a blade-like tool can be used in methods as aforesaid. Most preferably, the groove has a central plane and surfaces sloping upwardly from the sides of the groove towards the central plane. These sloping surfaces will tend to guide a lead engaged with the tool towards the central plane of the groove.
Yet another aspect of the invention provides methods of making semiconductor components. Methods according to this aspect of the invention include the steps of providing one or more conductive leads, each lead having an elongated connection section. The method further includes the step of forming a dielectric supporting structure incorporating one or more gaps aligned with the connection sections of the leads so that each lead is permanently secured to the supporting structure of one end of the connection section and releasably secured to the supporting structure of the other end of the connection section. The leads may be provided on a sheet-like dielectric support layer and may be supported by such layer. The step of forming the support structure may include the step of selectively removing a part of the dielectric layer to form a gap therein in alignment with the connection sections of the leads.
The step of providing the leads may include the step of forming each lead with a fragile section in the connection section. For example, the leads may be formed by providing strips of a conductive structural material so that each such strip has an interruption therein, and depositing a first supplemental material so that such supplemental material overlies each strip at least in a zone of the strip including the interruption, so as to leave portions of the strip on opposite sides of the interruption connected to one another by the first supplemental material. Thus, the frangible section of each lead may include a section formed from the supplemental material. The structural material and the supplemental material may both be metals and the supplemental material may be applied an a thin layer in a plating process. The dielectric layer may be formed from a polymeric material such as polyimide and the step of selectively removing a portion of the dielectric layer may be performed after forming the strips. That is, the strips are deposited on the dielectric sheet and the dielectric sheet is then etched or otherwise selectively treated so as to form the gap or gaps. After formation of the gap or gaps, one end of each connection section remains connected to the dielectric layer only through the frangible section, and hence is releasably connected to the dielectric sheet.
Alternatively, the leads may be formed by depositing strips of a conductive material on the dielectric sheet and then etching the dielectric sheet to form the gap or gaps. The dimensions of the gap or gaps so formed are controlled so as to leave each lead with a relatively large first end securement section bonded to the dielectric sheet on one side of the gap and with a relatively small, second end securement section bonded to the sheet on the other side of the gap, so that the end of each lead adjacent such other section can be detached from the dielectric sheet by breaking this relatively small bond. In this instance, there is no need to form a frangible section in each lead.
The foregoing and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.